Minimally, optical spectroscopy systems typically comprise a source for illuminating a target, such as a material sample, and a detector for detecting the light from the target. Further, some mechanism is required that enables the resolution of the spectrum of the light from target. This functionality is typically provided by a spectrally dispersive element.
One strategy uses a combination of a broadband source, detector array, and grating dispersive element. The broadband source illuminates the target in the spectral scan band, and the signal from the target is spatially dispersed using the grating, and then detected by an array of detectors.
The use of the grating, however, requires that the spectroscopy system designer make tradeoffs. In order to increase the spectral resolution of these systems, aperturing has to be applied to the light provided to the grating. As more spectral resolution is required, more light is required to be rejected by the narrowing spatial filter. This problem makes this strategy inappropriate for applications requiring a high degree of spectral precision combined with sensitivity.
Another approach is to use a tunable narrowband source and a simple detector. A typical approach relies on a tunable laser, which is scanned over the scan band. By monitoring the magnitude of the tunable laser's signal at the detector, the spectrum of the sample is resolved. These systems have typically been complex and often had limited wavelength scanning ranges, however.
Still another approach uses light emitting diodes (LEDs) and an acousto-optic modulator (AOM) tunable filter. One specific example combines multiple light emitting diodes (LEDs) in an array, each LED operating at a different wavelength. This yields a relatively uniform spectrum over a relatively large scan band. The light from the diodes is then sent through the AOM tunable filter, in order to create the tunable optical signal.
The advantage of this system is the use of the robust LED array. This provides advantages over previous systems that used other broadband sources, such as incandescent lamps, which had limited operating lifetimes and high power consumption.
While representing an advance over the previous technology, the disadvantages associated with this prior art system were related to the use of the AOMs, which are relatively large devices with concomitantly large power consumptions. Moreover, AOMs can also be highly temperature sensitive and prone to resonances that distort or alter the spectral behavior, since they combine a crystal with a radio frequency source, which establishes the standing wave in the crystal material to effect the spectral filtering.
Grating-based spectrometers also tend to be large devices. The device packages must accommodate the spatially dispersed signal from the sample. Further, the interface between the grating and the detector array must also be highly mechanically stable. Moreover, these grating based systems can be expensive because of costs associated with the detector arrays or slow if mechanical scanning of the detector or grating is used.